Emission of pollutants, for example, mercury, from sources such as coal-fired and oil-fired boilers has become a major environmental concern. Mercury (Hg) is a potent neurotoxin that can affect human health at very low concentrations. The largest source of mercury emission in the United States is coal-fired electric power plants. Coal-fired power plants account for between one-third and one-half of total mercury emissions in the United States. Mercury is found predominantly in the vapor-phase in coal-fired boiler flue gas. Mercury can also be bound to fly ash in the flue gas.
On Dec. 15, 2003, the Environmental Protection Agency (EPA) proposed standards for emissions of mercury from coal-fired electric power plants, under the authority of Sections 111 and 112 of the Clean Air Act. In their first phase, the standards could require a 29% reduction in emissions by 2008 or 2010, depending on the regulatory option chosen by the government. In addition to EPA's regulatory effort, in the United States Congress, numerous bills recently have been introduced to regulate these emissions. These regulatory and legislative initiatives to reduce mercury emissions indicate a need for improvements in mercury emission technology.
There are three basic forms of Hg in the flue gas from a coal-fired electric utility boiler: elemental Hg (referred to herein by the symbol Hg0); compounds of oxidized Hg (referred to herein by the symbol Hg2+); and particle-bound mercury. Oxidized mercury compounds in the flue gas from a coal-fired electric utility boiler may include mercury chloride (HgCl2), mercury oxide (HgO), and mercury sulfate (HgSO4). Oxidized mercury compounds are sometimes referred to collectively as ionic mercury. This is because, while oxidized mercury compounds may not exist as mercuric ions in the boiler flue gas, these compounds are measured as ionic mercury by the speciation test method used to measure oxidized Hg. The term speciation is used to denote the relative amounts of these three forms of Hg in the flue gas of the boiler. High temperatures generated by combustion in a coal boiler furnace vaporize Hg in the coal. The resulting gaseous Hg0 exiting the furnace combustion zone can undergo subsequent oxidation in the flue gas by several mechanisms. The predominant oxidized Hg species in boiler flue gases is believed to be HgCl2. Other possible oxidized species may include HgO, HgSO4, and mercuric nitrate monohydrate (Hg(NO3)2.H2O).
Gaseous Hg (both Hg0 and Hg2+) can be adsorbed by the solid particles in boiler flue gas. Adsorption refers to the phenomenon where a vapor molecule in a gas stream contacts the surface of a solid particle and is held there by attractive forces between the vapor molecule and the solid. Solid particles are present in all coal-fired electric utility boiler flue gas as a result of the ash that is generated during combustion of the coal. Ash that exits the furnace with the flue gas is called fly ash. Other types of solid particles, called sorbents, may be introduced into the flue gas stream (e.g., lime, powdered activated carbon) for pollutant emission control. Both types of particles may adsorb gaseous Hg in the boiler flue gas.
Sorbents used to capture mercury and other pollutants in flue gas are characterized by their physical and chemical properties. The most common physical characterization is surface area. The interior of certain sorbent particles are highly porous. The surface area of sorbents may be determined using the Brunauer, Emmett, and Teller (BET) method of N2 adsorption. Surface areas of currently used sorbents range from 5 m2/g for Ca-based sorbents to over 2000 m2/g for highly porous activated carbons. EPA Report, Control of Mercury Emissions From Coal-Fired Electric Utility Boilers, Interim Report, EPA-600/R-01-109, April 2002. For most sorbents, mercury capture often increases with increasing surface area of the sorbent.
Mercury and other pollutants can be captured and removed from a flue gas stream by injection of a sorbent into the exhaust stream with subsequent collection in a particulate matter control device such as an electrostatic precipitator or a fabric filter. Adsorptive capture of Hg from flue gas is a complex process that involves many variables. These variables include the temperature and composition of the flue gas, the concentration of Hg in the exhaust stream, and the physical and chemical characteristics of the sorbent. Of the known Hg sorbents, activated carbon and calcium-based sorbents have been the most actively studied.
Currently, the most commonly used method for mercury emission reduction is the injection of powdered activated carbon into the flue stream of coal-fired and oil-fired plants. Currently, there is no available control method that efficiently collects all mercury species present in the flue gas stream. Coal-fired combustion flue gas streams are of particular concern because their composition includes trace amounts of acid gases, including SO2 and SO3, NO and NO2, and HCl. These acid gases have been shown to degrade the performance of activated carbon. Though powdered activated carbon is effective to capture oxidized mercury species such as Hg+2, powdered activated carbon (PAC) is not as effective for elemental mercury which constitutes a major Hg species in flue gas, especially for subbituminous coals and lignite. There have been efforts to enhance the Hg0 trapping efficiency of PAC by incorporating bromine species. This, however, not only introduces significantly higher cost, but a disadvantage to this approach is that bromine itself is a potential environmental hazard. Furthermore, the presence of PAC hinders the use of the fly ash for cement industry and other applications due to its color and other properties.
As noted above, alternatives to PAC sorbents have been utilized to reduce mercury emissions from coal-fired boilers. Examples of sorbents that have been used for mercury removal include those disclosed in United States Patent Application Publication No. 2003/0103882 and in U.S. Pat. No. 6,719,828. In United States Patent Application Publication No. 2003/0103882, calcium carbonate and kaolin from paper mill waste sludge were calcined and used for Hg removal at high temperatures above 170° C., preferably 500° C. U.S. Pat. No. 6,719,828 teaches the preparation of layered sorbents such as clays with metal sulfide between the clay layers and methods for their preparation. The method used to prepare the layered sorbents is based on an ion exchange process, which limits the selection of substrates to only those having high ion exchange capacity. In addition, ion exchange is time-consuming and involves several wet process steps, which significantly impairs the reproducibility, performance, scalability, equipment requirements, and cost of the sorbent. For example, a sorbent made in accordance with the teachings of U.S. Pat. No. 6,719,828 involves swelling a clay in an acidified solution, introducing a metal salt solution to exchange metal ions between the layers of the clay, filtering the ion exchanged clay, re-dispersing the clay in solution, sulfidation of the clay by adding another sulfide solution, and finally the product is filtered and dried. Another shortcoming of the process disclosed in U.S. Pat. No. 6,719,828 is that the by-products of the ion exchange process, i.e., the waste solutions of metal ions and hydrogen sulfide generated from the acidic solution, are an environmental liability.
There is an ongoing need to provide improved pollution control sorbents and methods of their manufacture. It would be desirable to provide sorbents containing metal sulfides on the sorbent substrate that can be manufactured easily and inexpensively. In this regard, simple and environmentally friendly methods that effectively disperse metal sulfide on readily available substrates, which do not require the numerous steps involved in an ion exchange process are needed.